FixedWing

That's ok, we'll still let you volunteer. I actually don't think it makes a lot of difference, tip or not, so far as testing out the LSD, its handling characteristics and compatability with PSMis concerned.

Guess you might want to talk to Craig about getting another unit for the tip?

Stephen

03-turbo911

Talking to Craig can be hazardous to my bank account and general financial security.

I hereby decline the office of official LSD tester for the 996turbo forum.

cjv

Stephen,

The 40/60 will live with your PSM. I have tried it. In addition Todd at EVOMS has installed it in his car and a few others. You might want to give him a call.

Like I said before it can be a little tricky in a few situations. If you are an experienced driver you will have no issues and the positives far outweight the negatives. However, it wouldn't be a mod that you would want to hand to just anyone as they could get themselves into trouble at certain times when it engauges.

Craig.

Let me start by saying that I am among these least technically savvy guys on this forum and, therefore, my limited understanding of this subject is likely inaccurate in several respects. Nevertheless, I will attempt to post an important technical distinction worth discussing.

Torque/power tends to travel along the path of least resistance. If one tire has lost traction, but the other tire has retained traction, the tire that has lost traction will offer less resistance. Therefore, with an open differential, when a tire has lost traction and is therefore offering less resistance, that tire will receive a disproportionate amount of torque (more than the tire with traction), which may exacerbate the loss of traction. Meanwhile, the tire that has traction will receive a lower percentage of the torque because if offers greater resistance, even tough it has traction and the other tire does not. The undesirability of this disproportionate distribution of torque to the tire without traction is self-evident.

As I understand it, the above-referenced LSDs offered by Porsche Motorsports and Guard address this problem by locking the rear differential when a certain amount of wheel spin (loss of traction) is detected in one or both tires and, once the differential is locked, the rear wheels receive the same amount of torque (and turn at the same speed). The purpose of locking the differential and equalizing the torque is to prevent a disproportionate amount of torque from traveling along the path of least resistance to the wheel that has lost traction. The primary difference in the above-referenced LSDs is how much wheel spin/loss of traction is required to engage the LSD and lock the differential.

I am informed that the Quaife LSD is gear based (as opposed to clutch based) and works slightly differently. The Quaife LSD never locks the differential. Rather, when the Quaife unit detects loss of traction, it redistributes/biases the torque in an infinitely variable allocation between the two rear wheels (as opposed to the fixed 50/50 split when the differential is locked). Upon detecting loss of traction, the Quaife unit sends a greater amount of torque to the tire that has traction (and less to the tire that has lost traction). If the tire without traction continues to slip, the Quaife unit sends even more torque to the tire with traction (and further reduces the torque to the slipping tire). The Quaife LSD constantly adjusts the allocation of torque between the two rear tires on an infinite sliding scale until the slipping tire regains traction (or the tire with traction begins to slip). If a 60/40 allocation (60 to the tire with traction) enables the slipping tire to regain traction, the Quaife unit stops there, then eases back to 50/50 once traction is regained. Hypothetically, the Quaife LSD could distribute 90% of the torque to the tire with traction, and 10% to the slipping tire, although such an outcome is unlikely as the tire with traction would likely begin to slip before it reached 90% of the torque, and the Quaife unit would back off at that point.

Separately, Quaife advised me that they have 3 or 4 additional LSDs available for Tips, and many more for 996TT 6speeds.

Craig

Ps: Please remember my initial caveat: I am a complete novice when it comes to these technical distinctions, and I have likely mis-stated certain points in this post.

Zippy

Thanks guys for posting some excellent information on the + & - side of installing an LSD. I've been thinking about the merrits of an LSD with the additional HP my car has and find you comments invaluable.

1AS

Beware of differentials that increase traction thru "Locking". I have two cars with such differentials, and they require a different skill set for street driving.
When a locking diff actually locks (and in racing units you can hear it) the rear of the car must slide or drift, since the variable wheel speeds of the inside and outside tires are lost, and the inside wheel must traverse the same distance as the outer.
This is fine on a race car, where it helps rotate the car, and makes a light drift very typical. In a street car, you will fishtail every time you apply enough power to lock the diff. For example, when you drive off with a cold engine, and need to give it gas to keep it running, you will be immediately correcting a slide, if the steering wheel is not straight. This is not the same as a street "limited slip".
I think this is what CJV was referencing. (or not) AS

FixedWing

I must admit, I feel good about the responses I've had. I probably am going to try fitting one. Worse case, I pull it out again.

I do think you give good advice in warning that it isn't for everyone.

Thanx Chad.

FixedWing

You think Chad is talking about having a locker? That would make a lot of sense.

Yes, I've heard how nasty these can be -- especially when the lock on the overrun.

Stephen

FixedWing

What you describe sounds very similar to a torsen differential. I think it is pretty amazing how many different choices there are out there, each with its own unique characteristics.

thanx Craig.

Stephen

cjv

AS is correct. The street LSD is biased (clutch) geared while the mechaical LSD (as used in the Cup and RSR) is geared and will react as AS described. The mechanical units are designed for racing.

stuka

But why bother if you can't even fully turn off PSM. This is beginging to really p*ss me off. I even asked Ruf, and they apprently do not have a reprogramming kit to make PSM fully defeatable, in which case, real LSD's would totally make sense.

Until then, I really don't see the point. But hey, I would love to be wrong, since I would love to be able to drive my Turbo sideways just like I did my E46 M3.

stuka

If it actually locks, it might just fry the DME since PSM cannot be completely turned off, and part of its job is managing traction in the AWD.

Again, I would love to be wrong, and if installing these LSD's mean that I can finally get my Turbo sideways at will, then sign me up.

FixedWing

As I said on the PSM thread, I think you will find that it isn't PSM that is doing this but rather the basic handling characteristics of the car. PAG has intentionally made it an understeering car.

Stephen

FixedWing

Why would this fry anything? PSM takes its inputs from the engine, controls, accelerometers and individual wheels speed sensors. None of those would be affected. It acts through either cutting the power or by braking the wheels individually or in combination. Cutting power would not be affected. The only thing that would be affected would be the individual wheel braking of the rear wheels. This is because when the open differential is replaced with some sort of LSD differential braking one wheel could have some effect on the other rear wheel through the differential. This could either reduce its effectiveness or stop it from working depending on how high the overrun locking ratio was. If the differential locking ratio was not too high then the PSM might continue to work effectively. But no matter how efficient it still was, it wouldn't damage anything.

Stephen

03-turbo911

The new 997 turbo's coming out with the LSD option. Might need to wait and see what kind of unit that they're running on it.